A great deal of pre-clinical evidence and early clinical trial data suggests that the anti-tumor capabilities of the immune system can be harnessed to treat patients with established cancers. The vaccine strategy takes advantage of tumor antigens associated with various types of cancers. Immunizing with live vaccines such as viral or bacterial vectors expressing a tumor-associated antigen is one strategy for eliciting strong CTL responses against tumors.
Listeria monocytogenes (Lm) is a gram positive, facultative intracellular bacterium that has direct access to the cytoplasm of antigen presenting cells, such as macrophages and dendritic cells, largely due to the pore-forming activity of listeriolysin-O (LLO). LLO is secreted by Lm following engulfment by the cells and perforates the phagolysosomal membrane, allowing the bacterium to escape the vacuole and enter the cytoplasm. LLO is very efficiently presented to the immune system via MHC class I molecules. Furthermore, Lm-derived peptides also have access to MHC class II presentation via the phagolysosome.
Cancer is a complex disease and combined therapeutic approaches are more likely to succeed. Not only tumor cells, but also the microenvironment that supports tumor growth, must be targeted to maximize the therapeutic efficacy. Most immunotherapies focus on single antigens to target tumor cells and therefore they have shown limited success against human cancers. A single therapeutic agent capable of targeting tumor cells and tumor microenvironment simultaneously would have an advantage over other immunotherapeutic approaches, especially if it results in a synergistic anti-tumor effect.